1. Field of the Invention
This invention relates to polymerizable (curable) sealing and bonding (adhesive) compositions, and more particularly to compositions which contain polymerizable esters. The invention has particular application to polymerizable compositions which exhibit anaerobic curing characteristics, that is, compositions comprising polymerizable acrylate or methacrylate ester monomers and containing free radical polymerization initiators of the hydroperoxy or perester type and having the ability to remain in the fluid, unpolymerized state so long as contact with air or oxygen is maintained while at the same time being capable of rapid polymerization to the solid state upon the exclusion of air or oxygen, and will be described in connection with such application. (True anaerobics also polymerize rapidly in the presence of transition metal ions such as copper or iron.) It will be understood, however, that polymerizable compositions made in accordance with the instant invention may be cured by a variety of mechanisms in addition to anaerobic curing.
2. Prior Art
Various anaerobically curing polymerizable compositions are known in the art and are available commercially from a variety of manufacturers. Anaerobically curing compositions are commonly composed of curable unsaturated monomers, specifically acrylate or methacrylate ester monomers, in combination with one or more free radical polymerization initiators containing hydroperoxy moieties or materials that will hydrolyze in situ to hydroperoxy moieties, e.g. peresters. While the free radical polymerization initiators in the anaerobic composition are capable of polymerizing the monomers within a relatively short time (typically 30 seconds to ten minutes) on metal parts, for high speed production line applications where sealed or bonded articles cannot be stored for extended periods, and for equipment repair applications where prompt return to service is important, the highest possible rate of cure for the composition is desirable. (This assumes that the rapid cure speed does not produce overriding accompanying negative effects, such as undue embrittlement of the cured composition, or loss of other desirable properties.) Generally, for high speed production line applications the bonded parts should reach handling strength in 5 to 10 seconds. Handling strength means that the bonded parts can support their own weight by means of the cured adhesive and thus can be moved or subjected to further manufacturing processing without failure of the bond.
Of additional and particular importance in anaerobically curing compositions (as well as compositions curable by other mechanisms) is the ability to cure-through-gap. As the gap (i.e., the space between the substrates to be sealed or bonded) becomes larger, the possibility of entrainment of atmospheric oxygen increases, and the area of the adhesive which is in contact with oxygen also increases. Consequently, the inability to provide complete cure through large gaps has been a common problem with many anaerobic compositions.
It is thus a primary object of the present invention to provide an improved polymerizable sealing and bonding system (i.e. processes, materials and products) which overcome the aforesaid and other disadvantages of the prior art.
Other objects of the present invention are to provide improved anaerobically curing, sealing and bonding compositions, which compositions demonstrate extremely rapid fixture time, good adhesive strength, extended pot life and ease of application, and complete cure-through-large gaps, (i.e. in excess of 20 mil.). Yet other objects will in part appear obvious and will in part appear hereafter.
The invention accordingly comprises the processes involving the several steps and relative order of two or more of such steps with respect to each other, and the materials and compositions possessing the several features, properties and relations of elements which are exemplified by the following disclosure, and the scope of the application which will be indicated in the claims.